It is a common practice to manufacture mineral fibers, such as glass fibers, by supplying molten mineral material from a furnace or melter to fiber forming apparatus. The fiber forming apparatus commonly comprises spinners for centrifuging the molten mineral material into mineral fibers. Other fiber forming equipment used includes bushings or feeders for making continuous fibers of mineral material. When spinners are used for fiber forming, the production is sometimes facilitated by the use of burners which keep the spinner at proper operating temperatures. Also, annular blowers can be positioned adjacent the spinner to assist in attenuation of the fibers to the desired diameter.
One of the problems which has long plagued the manufacturers of mineral fibers is variation in the viscosity of the glass flowing into the fiber forming apparatus. In a continuous fiber manufacturing process, variations in the viscosity of the glass can affect both the yardage of the fiber glass, i.e., glass fiber diameter, and the strength of the glass fibers. In manufacturing wool type fibers using a rotary fiberizer, variations in glass viscosity can affect the throughput, the fiber diameters, and the length of the fibers.
Existing methods of glass viscosity measurement are not sufficient for adequate control. Typically, a glass sample is taken from the furnace or forehearth and the sample is removed to a testing station where the viscosity is determined using calibrated instruments. The process takes a considerable amount of time, and is done off line. The results of the viscosity analysis are often learned too late to make any correction in the fiber forming process.
Chrisman in U.S. Pat. No. 4,277,274, discloses a method of controlling a glass melting furnace in which the viscosity is determined, and in which molten glass is added to the forehearth in response to deviations from glass viscosity setpoints. The viscosity in Chrisman is determined using sensed electrical conductivity of the glass flowing through the forehearth. Chrisman also suggests that the viscosity can be determined using a strain gauge on the shaft of the mixer which can be positioned within the forehearth. These solutions proposed by Chrisman involve some uncertainty because of the fact that the glass on which the viscosity is being measured is still within the forehearth. It has been found that measuring viscosity with equipment installed in the mainstream flow of molten glass, such as in the forehearth, is difficult due to the hostile environment within the molten glass for the equipment. The lack of access to the equipment gives the inevitable result that the equipment fails and the measuring process is out of control.
There is still a need for a method and apparatus which enables the glass viscosity to be measured on line, and in a continuous mode. It is also important that the viscosity measurement method be accomplished without disturbing the mineral fiber manufacturing process. Further, the equipment should be outside of the molten glass in order to give access and longevity to the equipment.